The present invention relates to the art of magnetic resonance imaging. It finds particular application in conjunction with motion desensitization of magnetic resonance images and will be described with particular reference thereto. It is to be appreciated, however, that the present invention may also be applicable to other image enhancement, modification, and improvement techniques.
Heretofore, medical diagnostic magnetic resonance imaging has included the sequential pulsing of radio frequency signals and magnetic field gradients across a region to be imaged. In two dimensional imaging, a patient is disposed with a region of interest in a substantially uniform main magnetic field. A slice select gradient is applied across the field to select a slice or other region of the patient to be imaged. A phase encode gradient is applied along one of the axes of the selected slice to encode the material with a selected phase angle along the phase encode axis. In each repetition of the pulse sequences, the phase encode gradient is commonly stepped in regular increments from a first peripheral phase encode angle of +90.degree. in increments through a central phase angle of zero to an opposite peripheral phase angle of about -90.degree.. Each repetition of the pulse sequence produces a corresponding set of sampled data points, generally termed a view or step. In this manner, each view is phase encoded with a corresponding one of the phase angle increments. The central or zero phase encoded views provide the contrast in the resultant image; whereas the views phase encoded near the peripheral angles contribute the fine detail or resolution. A frequency encode gradient pulse frequency encodes the material along another axis of the slice, conventionally perpendicular to the phase encode axis.
Various motions during the acquisition of magnetic resonance data degrade the resultant images. The motions may be divided into two groups--rapid motions which transpire within the time to collect one view and slower motions which continue to occur over the collection of several views. The rapid motions tend to degrade fine resolution or detail, but have little effect on contrast. The effects of rapid motions can be reduced using prior art gradient rephasing techniques or the like.
The slower, low frequency motions tend to cause ghosts and other contrast defects without degrading the fine detail. The effects of slower, low frequency motions, such as respiratory or body movement, are commonly reduced by averaging. That is, the set of pulse sequences that produce the set of view used to reconstruct an image is repeated a plurality of times to collect redundant data. The redundant views corresponding to the same phase angle are averaged and the averaged views are utilized to reconstruct the image representation. Heretofore, the same number of redundant views have been collected corresponding to every phase angle. Although the number of redundant views corresponding to each phase angle might be as low as two, larger numbers of views, such as eight, sixteen, or more, are not uncommon.
One of the drawbacks with the prior art view averaging techniques has been the extended scan time. To produce and average two sets of redundant views requires twice the scanning time of collecting a single set. Similarly, averaging eight or sixteen sets of redundant views increases the scan time by a factor of eight or sixteen respectively. Thus, reducing image degradation attributable to slower motions causes a corresponding increase in scan time. Correspondingly, shortening the scan time can be achieved by reducing the number of views averaged, but at the cost of greater sensitivity to slower motion artifacts and degradation.
The present invention provides a new and improved magnetic resonance data processing technique which enables slow or low frequency motion degradation to be reduced without increasing imaging time or, conversely, to reduce imaging time without increasing artifacts from slower motion.